1. Field of the Invention
The present invention is related to rotary motion sensors and, more particularly, is directed towards a rotary motion sensor which employs a multiple turn fiber interferometer.
2. Description of the Prior Art
Various devices have been developed which utilize a fundamental phenomenon, sometimes referred to as the Sagnac interferometer effect, the early exploratory work in this field having been accomplished by Sagnac, Michaelson and others. Sagnac demonstrated that light traversing a closed path experiences an apparent path length change when the closed path is rotated about an axis perpendicular to the plane containing the closed path. He demonstrated that the apparent path length increased in the direction of rotation and decreased in the opposite direction.
With the advent of the laser and coherent optical radiation, devices known as laser gyroscopes have been constructed based upon Sagnac's theory, and have been developed into precision instruments for measuring rotary motion. In a laser gyro, a closed optical circuit or ring is established by the laser and mirrors about which coherent light is propagated in two directions, clockwise and counter-clockwise. The path lengths about the ring are frequency determining parameters of the clockwise and counter-clockwise lasing. Increasing the path length by rotation causes a decrease in frequency, while decreasing the path length causes a frequency increase. The beat frequency between the clockwise and counter-clockwise laser beams is directly proportional to the rotational rate of the apparatus.
The laser gyro, however, has not been widely adopted as a result of several serious problems. One such problem is known as the lock-in problem which results from the tendency of the clockwise (CW) radiation and the counter-clockwise (CCW) radiation to lock together in frequency as the rotation of the gyro slows until it reaches a point where the radiation in the CW direction scatters from imperfections in the path and mixes with the CCW radiation, and vice versa. When lock-in occurs, the rotation rate can no longer be determined as a frequency difference. Many proposals have been advanced, as evidenced by, for example, U.S. Pat. Nos. 3,846,025; 3,841,758; and 3,714,607, whose primary objectives are to obviate the frequency locking problems of ring laser rotary motion sensors.
Up until recently, the interferometer did not receive much interest as a technique for measuring rotation, since it is not as sensitive as the laser gyro. However, with the development of glass and single crystal waveguides for use with optical and infrared coherent radiation, the interferometer having multiple optical turns has become a viable solution for precise rotation measurements. Experiments performed at the University of Utah by Vali and Shorthill have demonstrated the validity of this concept, as published in the June, 1975 issue of Laser Focus.
The concept as suggested by Vali and Shorthill, while demonstrating feasibility, nevertheless made no suggestion as to the manner of adopting the principle to perform as a practical, field worthy device which may be utilized, for example, in missiles for measuring rotation rate and attitude. Present techniques for accomplishing the latter, for example, require very expensive optical systems for tracking the missile by taking photographs which must later be manually reduced to obtain information as to the missile attitude versus time. In addition to being labor intensive, the manual data reduction takes many months to accomplish, and requires expensive labor and equipment.
It is therefore clear from the foregoing that it would be quite advantageous if the multiple turn laser interferometer principle could somehow be adapted for use into a rugged, field worthy device for use in missles, aircraft, and other vehicles with limited space and very rigorous environments. If adapted for use, for example, in tracking missle attitude versus time, such a device could achieve more accurate data, at longer ranges, and be far less expensive than presently used optical systems. The data could additionally be obtained far quicker than presently available.
One of the difficulties encountered in the utilization of a fiber interferometer for rotary motion sensing are the extremely high laser frequencies with which one must deal. In a missle-mounted system, it may be appreciated that it would be extremely advantageous if ordinary electronic instrumentation could be utilized to detect and process the measurement signals generated by such a device. However, the prior art does not suggest how this may be accomplished with lasing frequencies on the order of 10.sup.14 Hz.